Fine-grained mixtures can be used as additives or fillers for the construction or treatment of pavings or traffic surfaces, like asphalt-bound roadways. When such mixtures possess a salt content, their addition to the surface of a paving causes a lowering of the freezing point of water (rain, snow, sleet or the like), and thus in winter, contributes to an increase in traffic safety by preventing icing, especially at ambient temperatures fluctuating around 0.degree. C., and makes snow removal easier since the adhesion of snow to the pavement surface is definitely reduced.
It is essential that the additive has hydrophobic properties in order, on the one hand, to insure its workability (free-flowing property, no agglomeration) and, on the other hand, to maintain its freezing-point lowering action over very long periods of time despite the action of surface water.
Numerous methods are known for the preparation of salt-containing additives of similar action for roadway paving. In these methods, the salt, in dissolved form, is deposited in the voids of a porous carrier material or is deposited on a fibrous material and an additive, which has been mixed with a hydrophobizing agent, is obtained by drying and subsequent grinding. By way of example, reference can be made to Federal Republic of Germany Patent 25 12 591, European Patent 00 22 055, Federal Republic of Germany Patent 29 45 823, European Patent 00 48 417 and Federal Republic of Germany Patent 31 47 773. A disadvantage of these methods is the energy consumption necessary for drying.
This disadvantage can be remedied by means of a method disclosed in European Patent 0,153,269 which belongs to the same generic class as the present invention but differs in some important respects. This prior art method avoids drying of the additive, since the hydrophilic component (salt and possibly powdered lava and/or powdered quarts) and the hydrophobic component (powdered foamed polyurethane resin and/or carbon black and/or calcined perlite) of this additive, are combined in dry form and ground together without using a liquid intermediate phase. The fine-grained mixture produced in this way, having a grain size of less than 0.2 mm, and preferably less than 0.1 mm, is therefore free from recrystallization products or gels produced from the combination of the components, in contrast to the result obtained when working with a liquid phase.
In the preparation of the fine-grained mixture of the above prior art type, the content of the required salt (in particular sodium chloride) represents a substantial cost factor. An attempt is therefore made to use waste salts (e.g., from potassium production) costing as little as possible for this purpose, especially since there are no special requirements for the purity of the salt per se for this application. It need only be free of environmentally harmful accompanying substances.
Since, however, the cost is still considerable for waste salts from potassium production, it is desirable to seek additional cost-lowering alternatives. One such possible alternative is the use of residual salts such as those resulting from the production of glycerol, by distillation of spent-spent-lye crude glycerol from soap production. These salts contain essentially sodium chloride and are contaminated with accompanying substances.
The use of such residual salts from glycerol production in the method of this type, however, involves difficulties. For one thing, it is a concern that the residual soap content present with the salt will dissolve in the presence of moisture, leading to unwanted and unacceptable formation of a slick on the wet roadway paving. For another, test have shown that in this type of method this residual salt does not lead to a fine-grained mixture with hydrophobic properties. The use of other highly effective but extremely expensive hydrophobizing agents, even when they are added in large quantities, results in no hydrophobization, or at least inadequate hydrophobization, of the fine-grained mixture. To date, therefore, these residual slats have not been suitable for the preparation of icing-inhibiting paving additives either from the standpoint of the desires cost-reducing effect (cost increase due to expensive hydrophobizing agents) or from the technical standpoint.
It is therefore the object of the invention to make possible the use of residual salts, in particular of the aforementioned type from glycerol production, for the preparation of a filler for pavements and traffic surfaces, that is, in particular, to provide a method in which a satisfactory hydrophobization of the fine-grained mixture can be achieved at low cost.